[wp-trac] [WordPress Trac] #58323: Use static closures whenever $this is not used to avoid memory leaks

[WordPress Trac]

#58323: Use static closures whenever $this is not used to avoid memory leaks
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 Reporter:  westonruter  |      Owner:  (none)
     Type:  enhancement  |     Status:  new
 Priority:  normal       |  Milestone:  Awaiting Review
Component:  General      |    Version:
 Severity:  normal       |   Keywords:
  Focuses:  performance  |
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 Historically closures were not usable in WordPress since they were
 introduced in PHP 5.3 and WordPress supported PHP 5.2. However, now
 WordPress requires PHP 5.6 so closures have started making their way into
 the codebase. Whenever a closure is used inside of a class instance, the
 `$this` is automatically captured by the closure. If the closure is added
 as a filter and not removed, for example, the class instance won't be able
 to be garbage collected even if there are otherwise no other references.
 For any such closures that don't reference `$this`, this can be avoided by
 adding a `static` keyword to the closure. Doing so prevents `$this` from
 being captured and thus avoids a memory leak. See
 [https://stackoverflow.com/questions/19899468/php-closures-why-the-static-
 in-the-anonymous-function-declaration-when-bindin/64481894#64481894 Stack
 Overflow answer].

 This can be avoided in the future by incoporating the
 [https://github.com/slevomat/coding-standard
 SlevomatCodingStandard.Functions.StaticClosure] sniff.

 Also implemented in [https://github.com/WordPress/performance/pull/729 PR
 for Performance Lab plugin].

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Ticket URL: <https://core.trac.wordpress.org/ticket/58323>
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